A thermionic converter converts thermal energy into electric energy utilizing a phenomenon in which thermions are emitted from a surface of an electrode at a high temperature.
According to a non-patent document 1, when a diamond (diamond semiconductor) is used for an emitter (emitter electrode) and a collector (collector electrode) of a thermionic converter, a work function reduces and thermions are efficiently emitted from a surface of the electrode due to an effect of negative electron affinity (NEA).
In order to inject or extract electrons into or from the diamond at a high efficiency, it is necessary to form an ohmic contact. To form the ohmic contact in the electrode using the diamond, generally, a metal is vapor-deposited in diamond and then annealing is performed at a high temperature. Thus, a reaction layer is formed at an interface between the metal and the diamond.
[Non-Patent Document 1]
F. A. M. Koeck, Y. j. Tang, R, j. Nemanich, Organizing Committee NDNC2007, NDNC 2007 New Diamond and Nano Carbons 2007, May 28, 2007, p 97, “Direct thermionic energy conversion from nitrogen doped diamond films”, North Carolina State University, Raleigh, N.C., USA, Arizona State University, Tempe, Ariz., USA
However, when the diamond is used as a material of an electrode, such as an emitter electrode, of the thermionic converter, the following drawbacks arise:
In order to improve the efficiency of thermions, a method of terminating the surface of the diamond layer of the emitter electrode with hydrogen atoms has been known. As an example of the hydrogen termination method, for example, a diamond layer is formed by a microwave chemical vapor deposition (CVD) technique, and then carbon atoms on the surface of the diamond layer are terminated by hydrogen separation through a hydrogen plasma treatment in the same chamber. As a result, the surface which has a negative electron affinity is formed.
However, when the metal is deposited on the hydrogen-terminated diamond layer and is annealed to form the reaction layer for the ohmic contact, the bonding between the carbon atom and the hydrogen atom on the surface of the diamond layer is likely to be broken and the hydrogen atom is likely to be separated. As a result, the work function increases and the negative electron affinity reduces. Further, the emission of the thermions from the surface deteriorates.
In regard to the collector electron, similarly, when the reaction layer is formed after the hydrogen termination in the similar manner, the hydrogen atoms are likely to be separated, resulting in an increase in work function.
Accordingly, in the emitter electrode and the collector electrode manufactured by the above described method, the power generation efficiency of the thermionic converter is likely to be reduced.